Objective: This study aimed to investigate the effect of different-sized airways of the inspiratory muscle trainer (IMT) on maximal inspiratory pressure (MIP) and the rating of perceived exertion, as measured by the modified Borg scale (m-Borg). Design: Cross-sectional study. Methods: Twenty healthy subjects (10 men, 10 women) volunteered for the study. The spirometry was used to measure MIP. The trial order of the three spirometry conditions was chosen randomly. After measuring the MIP and before taking the final break, each of the conditions were immediately measured using the m-Borg. All subjects used the IMT with an airway diameter of 5-, 6-, and 7-mm. Results: The MIP significantly decreased with the decreasing airway diameter of the IMT (p<0.001), and the differences in all three conditions (7- and 6-mm, 6- and 5-mm, and 7- and 5-mm airways) were significant (p<0.05). The RPE significantly increased with the decreasing airway diameter of the IMT (p<0.001), and the differences in all three conditions (7- and 6-mm, 6- and 5-mm, and 7- and 5-mm airways) were significant (p<0.05). Conclusions: Decreasing the airway diameter of the IMT decreased the MIP and increased the m-Borg. In regards to physical exercise within the clinical setting, the m-Borg could be a useful as a prior monitoring method to prevent dyspnea for patients with narrowed airways (blocked with secretion or sputum in pulmonary disease) and the different-sized airways of the IMT could be applied as a useful tool to improve MIP for prevention of pulmonary disease.